1. Field of the Invention
The present invention relates to a microphone apparatus to be built into an appliance possessing a mechanical system generating noise or mechanical vibration in its enclosure.
2. Description of the Prior Art
In the downsizing trend of appliances, such as video cameras and cassette tape recorders, possessing a recording function, microphones installed in such appliances have been changed from a type projecting from an enclosure of an appliance, to a type built in a small space provided in a part of the enclosure. However, for example, in the case of a video camera, since various mechanical systems are incorporated in its enclosure such as a tape running system for recording and a lens driving system for providing a zooming function, noise or mechanical vibration generated by the mechanical systems are transmitted to a built-in microphone, and a signal-to-noise ratio is significantly lowered when picking up sound.
A microphone apparatus for reducing the noise generated by the mechanical systems incorporated in the appliance by using an adaptive filter has been already proposed (for example, see Japanese Laid-Open Patent Application No. 3-295400). The microphone apparatus comprises a primary microphone, a vibration pickup unit provided in a mechanical system comprising a noise source, and an adaptive signal processing unit for reducing a noise mixed into an audio signal using a detected signal as a reference signal. The thus composed microphone apparatus mimics an impulse response of a transmission path from a vibration source to the primary microphone by the adaptive filter, and a false noise generated by convoluting the impulse response having mimicked a signal detected by the vibration pickup unit is added in opposite phase to an output signal of the primary microphone. The noise generated by the mechanical systems may be classified into two types, that is, noise component radiated into the air and transmitted to the primary microphone as a sound wave, and a vibration noise component coupled to the primary microphone by vibration transmitted through the enclosure, and both are caused by vibration of the mechanical systems. It is therefore possible to reduce the noise component contained in the output signal of the primary microphone, by directly detecting the vibration of the mechanical systems comprising a noise source, and using the detected signal as the reference signal.
In the case of the microphone apparatus mentioned above:
In the appliance possessing a recording function, usually, numerous mechanical parts are mounted at a high density in the enclosure, so a structure in the enclosure is extremely complicated. Therefore, to use vibration of a vibration source remote from the primary microphone directly as the reference signal, it is necessary to extend an impulse response length for mimicking with the adaptive filter, heighten a sampling frequency in order to express complicated transfer characteristics, and increase a number of taps of the adaptive filter. PA1 Moreover, generally, plural vibration sources are present in the enclosure, and in order to suppress the noise corresponding to each of the vibration sources, a number of vibration detecting means and a number of the adaptive signal processing units each equal to a number of the vibration sources should be required.
Because of the above reasons, a large amount of hardware was needed for the adaptive signal processing unit.